1. Field of the Invention
The present invention relates to a layered stripline filter, and more particularly to a layered stripline filter for use as a high-frequency circuit filter in a high-frequency radio communication device such as a portable telephone set or the like, or in an antenna duplexer, or the like.
2. Description of the Prior Art
FIGS. 1 and 2 of the accompanying drawings show, respectively in exploded perspective and perspective, a conventional layered stripline filter devised by the inventors of the present application.
As shown in FIG. 1, the layered stripline filter has a dielectric layer 11, a dielectric layer 13 disposed on the dielectric layer 11 and supporting an output electrode 42 thereon, and a dielectric layer 14 which is placed on the dielectric layer 13 and supports thereon a plurality of resonant elements 21, 22, 23 each composed of a quarter-wave stripline resonator, the resonant elements 21, 22, 23 having ends connected to a ground electrode 70 (see FIG. 2), the output electrode 42 underlying a portion of the resonant element 23 on an output terminal side across the dielectric layer 14. The dielectric layer 14 also supports thereon a plurality of electrodes 31, 32, 33 having ends connected to the ground electrode 70 and opposite ends spaced predetermined distances from the open ends of the resonant elements 21, 22, 23, respectively, in confronting relationship thereto. The layered stripline filter also includes a dielectric layer 15 positioned on the dielectric layer 14 and supporting thereon an input electrode 41 which is positioned in overlapping relationship to the resonant element 21 on an input terminal side across the dielectric layer 15, and a dielectric layer 17 placed on the dielectric layer 15, with the ground electrode 70 disposed on a surface of the dielectric layer 17. The dielectric layers 11, 13, 14, 15, 17 are integrally combined and then fired into a laminated assembly 500 (see FIG. 2).
As shown in FIG. 2, the ground electrode 70 is disposed on upper and lower surfaces of the laminated assembly 500 and side surfaces thereof except input and output terminal areas 61, 62. The input terminal area 61, which is positioned on one side surface of the laminated assembly 500, has an input terminal 51 that is insulated from the ground electrode 70 and connected to the input electrode 41. The output terminal area 62, which is positioned on an opposite side surface of the laminated assembly 500, has an output terminal 52 that is insulated from the ground electrode 70 and connected to the output electrode 42.
FIG. 3 of the accompanying drawings shows an equivalent electric circuit of the layered stripline filter shown in FIGS. 1 and 2. In FIG. 3, the equivalent electric circuit includes a capacitance 111 between the resonant element 21 and the input electrode 41, a capacitance 112 between the resonant element 23 and the output electrode 42, a capacitance 121 between the resonant element 21 and the electrode 31, a capacitance 122 between the resonant element 22 and the electrode 32, a capacitance 123 between the resonant element 23 and the electrode 33, an inductance 132 indicative of inductive coupling between the resonant elements 21, 22, and an inductance 133 indicative of inductive coupling between the resonant elements 22, 23. The equivalent electric circuit of such an arrangement serves as a bandpass filter. The equivalent electric circuit also includes parallel resonant circuits having respective capacitances 211, 221, 231 and respective inductances 212, 222, 232 which are equivalently converted from the respective resonant elements 21, 22, 23.
The electrodes 31, 32, 33 are disposed in confronting relationship to the open ends of the resonant elements 21, 22, 23, respectively. Therefore, the capacitances 121, 122, 123 are formed between the open ends of the resonant elements 21, 22, 23 and the electrodes 31, 32, 33. These capacitances 121, 122, 123 are added respectively to the capacitances 211, 221, 231 as equivalently converted from the respective resonant elements 21, 22, 23. If the parallel resonant circuits have the same resonant frequency, then the inductances of the parallel resonant circuits may be smaller, the resonant elements 21, 22, 23 may be shorter, and hence the overall length of the layered stripline filter may be reduced.
If the electrical length of the resonant elements is reduced to reduce the size of the layered stripline filter, however, the resonant elements are coupled by stronger inductive coupling, resulting in a filter bandwidth that is too wide. Consequently, it is not possible to achieve a layered stripline filter having a desired bandwidth.
With the layered stripline filter of the above structure, the electromagnetic fields of the resonant elements 21, 22, 23 are disturbed at short-circuited portions thereof, tending to intensify the inductive coupling thereof. Accordingly, the bandwidth of the layered stripline filter is undesirably increased by the strong inductive coupling between the resonant elements.